This invention relates generally to titanium-based alloys utilized in high-performance applications. More specifically, it relates to titanium-based alloys which are inspected for defects by employing ultrasonic techniques.
Titanium-based alloys are very useful materials because of their attractive combination of high strength and relatively low weight, at temperatures up to about 550xc2x0 C. They are therefore the material of choice for high performance components, such as compressor discs for aircraft propulsion systems. A wide range of alloys are available, each conferring a particular combination of characteristics to the component. In terms of end use applications, titanium alloys have replaced steel in the 200xc2x0 C.-500xc2x0 C. use temperature, and can often replace cobalt- and nickel-base alloys at higher temperatures.
The microstructure of titanium alloys has been the object of extensive research. It""s well understood that many important titanium alloys consist of at least two phases: an alpha (xcex1) phase which has a hexagonal close packed crystal structure, and a beta (xcex2) phase which is body-centered cubic. Transformation from xcex1 to xcex2 (in pure titanium) is known to occur at a temperature of about 880xc2x0 C. Alloying elements are frequently used to alter the xcex1-xcex2 transformation temperature. For example, aluminum, tin, and zirconium are common xcex1xe2x80x94stabilizing elements, while vanadium, molybdenum, and tantalum are common xcex2xe2x80x94stabilizing elements. Thermomechanical processing techniques for converting a cast billet of titanium into a finished article are generally well-known in the art.
Titanium alloys used for aerospace components must be of the highest quality. It is therefore necessary to inspect the alloy during various stages, such as the billet stage. As described in U.S. Pat. No. 5,533,401 (R. Gilmore), titanium billets are often formed from cylindrical ingots having a diameter of 30-36 inches, and a weight in the range of 7000-10,000 pounds. The ingots can be forged into a series of cylindrical billets, which can vary from 6 to 15 inches in diameter. Individual billet segments often are 10-20 feet in length.
Various types of nondestructive testing are available to determine the quality of the alloy, i.e., the type and amount of flaws within its microstructure. Examples of test methods include neutron imaging, electron imaging, and ultrasonic examination. Ultrasonic testing is one exemplary test method, in which ultrasonic tests are performed by introducing beams of high frequency sound waves into the material under investigation. Test signals indicate amplitudes and arrival times of transmitted, reflected, and refracted waves. The signals can also detect various interfaces and internal discontinuities within the object, e.g., grain boundaries, voids, cracks; and inclusions of foreign material, such as hard alpha inclusions. An ultrasonic transducer usually serves as both a generator of the ultrasonic beam and a detector of the attenuated sound waves which are produced from surfaces and interior discontinuities within the object. These sound waves are converted into electrical signal oscillations for inspection.
One particular type of ultrasonic technique is referred to as immersion testing, where the object being examined is submerged in a tank of liquid, and the sound beam from an ultrasonic transducer interrogates the test object. While many variations and additional accouterments are commercially available, the same general principles of ultrasonics are employed.
In the referenced patent of R. Gilmore, a multi-zone ultrasonic apparatus is described. Such a device is very useful for inspecting the entire volume of an object, such as a titanium billet. The device usually employs a plurality of ultrasonic transducers having focal zones at increasing depths, with adjacent focal zones overlapping each other. Additional features include a system for collecting data so that the billet can be examined as a series of C-scan images generated from each of the transducer signals.
The presence of xe2x80x9cmicrostructural noisexe2x80x9d in objects such as those made from titanium alloys can sometimes limit the ability of the ultrasonic apparatus to detect flawsxe2x80x94even when using a multi-zone ultrasonic apparatus. This situation is described by S. Foister et al in xe2x80x9cAn Experimental Investigation of Ultrasonic xe2x80x98Grain Noisexe2x80x9d in Titanium-6AI-4Vxe2x80x3; Review of Progress in Quantitative Nondestructive Evaluation; Plenum Press, Vol. 15B, pp. 1479-1486 (1996). While ultrasonic pulses are desirably reflected by bona fide flaws in the material, they can also reflect off benign features, such as grain boundaries.
Although the grain boundary reflections (i.e., xe2x80x9cgrain noisexe2x80x9d) are usually characterized by a distinct amplitude, at least two problems arise from the presence of intrinsic microstructural noise. First, the smallest flaw signals cannot be observed because they are masked by the grain noise, thereby limiting the detection capability of the ultrasonic scan. Second, the largest noise signals may be mistaken for flaws, resulting in numerous xe2x80x9cfalse callsxe2x80x9d, which in turn can lead to the rejection of good material.
It should thus be apparent that further improvements in the ultrasonic detection of flaws in titanium-based alloys would be welcome in the art. Moreover, the discovery of titanium-based materials which intrinsically exhibit a high level of ultrasonic xe2x80x9cinspectabilityxe2x80x9d would also represent a very significant advance in technology. Such materials should be amenable to a variety of different types of ultrasonic inspection techniques. The materials should also continue to exhibit, in their final form for use, substantially all of the properties sought after in titanium alloys, such as tensile strength, corrosion resistance, and fatigue crack growth resistance.
As disclosed in PCT Application WO 98/17836, several methods are disclosed for forming titanium, as embodied by the invention. For example, the titanium material that is subject to the inspection, as embodied by the invention, can be formed from nay one of the processes set forth in the Examples of the PCT Application WO 98/17836. Further, the titanium material that is subject to the inspection, as embodied by the invention, can be formed by other processes. For example, A method for preparing a titanium alloy article in which the titanium comprises a substantially controlled homogeneous fine grain microstructure. The method comprises the steps of: providing a titanium alloy article having an initial grain size (do); selecting a final homogeneous fine grain size (dk) to be achieved in the titanium alloy article; plotting a curve of the relationship between a recrystallized grain size (d) for the titanium alloy on the y-axis versus a strain temperature (T) for the alloy on the x-axis, between a range of 400xc2x0 C. and a temperature of complete polymorphous transformation (Tcpt), in accordance with the relationship d=f(T); locating an area (T*) on the strain temperature axis to divide the temperature axis into two zones comprising a first zone 400xc2x0 C. to T*, and a second zone T* to Tcpt, where the T* is located by first calculating a corresponding recrystallization grain size (d*) on the y-axis, where d* is logarithmically related to the initial grain size do; further locating, on the curve, the final grain size (dk) on the y-axis and then a corresponding strain temperature (Tk) on the x-axis; determining the heating and deforming step or steps to process the article based on Tk, where for Tcpt greater than Tk greater than T*, and there is at least one heat and deforming step to obtain the final grain size dk, where Tk less than T*, there are at least two heat and deforming steps where each heat and deforming step occurs for a sufficient amount of time to reduce the grain size of the titanium alloy article until the final grain size dk is obtained; heating and deforming the titanium alloy article in accordance with the determined number of heat and deforming steps to achieve (dk), where each heat and deforming step has at least one heating and deforming step and one cooling step, where the heat and deforming step occurs for a sufficient period of time to reduce the grain size of the titanium alloy article, and where the deformation of the titanium alloy article is in a substantially controlled manner during each heat and deforming step at a rate of strain to achieve the desired grain size of the heat and deforming step, where the true strain during the deformation is greater than or equal to about 0.6 for each heat and deforming step, and where the subsequent cooling is controlled at a temperature below the heat and deforming step temperature at a cooling rate for substantially maintaining the reduced grain size obtained during the heat and deforming step; and repeating the step of heating and deforming the titanium alloy article in accordance with the determined number of heat and deforming steps to achieve homogeneous grain size(dk), until a final substantially controlled homogeneous grain size dk is obtained in the article having substantially homogeneous mechanical properties.
Another method for making a substantially controlled homogeneous fine grain microstructure in a titanium alloy article, as embodied by the invention, comprises the steps of: heating and deforming titanium material at a predetermined heat and deforming step temperature that is at or below a temperature of complete polymorphous transformation where the titanium alloy article has sufficient ductility and a starting grain size, in which the heat and deforming steps comprise at least one heat and deforming step and at least one cooling step. The heat and deforming steps is conducted for a sufficient amount of time to reduce the grain size from a starting grain size to a reduced grain size at the end of the heat and deforming step. The deforming of the titanium occurs in a controlled manner at a rate of strain that is able to achieve a desired grain size, where a true strain during the deformation is greater than or equal to about 0.6 for each heat and deforming step and where the cooling step is performed after the heat and deforming step at a temperature below the heat and deforming step temperature, in a controlled manner at a cooling rate to substantially maintain the reduced grain size obtained during the heat and deforming step. The method also includes continuing to heat and deform then cool the titanium alloy article, in which the heating occurs at lower heat and deforming step temperatures than the previous heat and deforming step temperature, so a reduction of grain size is achieved in subsequent heat and deforming steps until a final controlled grain size with controlled mechanical properties is obtained in the titanium alloy article.